The present invention relates to an apparatus for isotope separation or mass analysis by a magnetic field. It applies to the isotope separation of the ions of a mixture of isotopes or to mass analysis.
It is known that the phenomenon of cyclotron resonance obtained by the electromagnetic effect was firstly observed on the electrons of the first cyclotrons. The application of cyclotron resonance to the ions of the isotopes of uranium for the enrichment of one of these isotopes was used in the case of the Manhattan project. Rapid advances in plasma physics led to research being taken up again as from 1970 in France and then in the U.S.A., where it led to the construction by the TRW company of an apparatus for isotope separation by said process using a uniform magnetic field. This apparatus is described in the journal "annales des mines", February/March 1983 in an article entitled "Research and development of uranium isitope separation in France" (original in French), pp 13 and 14. Isotope separation by this longitudinally extending apparatus is obtained by energy transfer between an electromagnetic wave and ions.
In a similar process, the perpendicular energy of the ions is increased as a result of the resonance effect observed, under certain conditions, when the particles are injected into a magnetic field which is periodically spatially modulated along the axis of the apparatus. Such a process is particularly by described in "Journal de physique lettres", No. 46, 15.8.1985, pp. L745 to L749 in an article entitled "Heating particles in a periodic magnetic structure" (original in French).
It can be gathered from this article that the particles describe a helical trajectory, whose diameter increases exponentially with time. Experimental and theoretical studies reveal that an essential condition for permitting a longitudinal energy transfer (parallel to the axial field) to transverse energy is a parametric resonance condition given by the relation V.sub..parallel.o =.OMEGA..sub.co (.lambda..sub.o /2.pi.), in which:
.OMEGA..sub.co designates the cyclotron pulsation of the ions considered (inverse of the time taken by an ion to perform a complete revolution, i.e. 2 radians on its trajectory in a mean amplitude field B.sub.O); PA1 V.sub..parallel.o designates the mean velocity of the resonant ions parallel to the direction of the modulation magnetic field (axial field), said field having a mean amplitude B.sub.O ; PA1 .lambda..sub.O designates the wavelength of the spatial modulation of the magnetic field in a longitudinal direction (parallel to the axis of the apparatus).
In a strictly regular and cylindrical, periodic magnetic structure, the shape of the magnetic field B.sub.z parallel to the axis Z of said structure is e.g. written: EQU B.sub.z =B.sub.o (1+.epsilon. sin k.sub.o .multidot.z)
with k.sub.o =2.pi./.lambda..sub.o.
In this relation, as hereinbefore B.sub.o designates the mean component of the axial magnetic field B.sub.z and .epsilon. designates the half-amplitude of the maximum variation of the component of the axial magnetic field B.sub.z about the mean amplitude B.sub.o of said field. Thus, when k.sub.o .multidot.z=1, it is possible to write: EQU B.sub.zmax =B.sub.o .multidot.(1+.epsilon.)
However, when k.sub.o .multidot.z=-1, the minimum value of the axial magnetic field can be written: EQU B.sub.zmin =B.sub.o .multidot.(1-.epsilon.)
Thus, the amplitude of the maximum variation of the component of the magnetic field about the mean component B.sub.o of said field has the value: EQU .DELTA.B=B.sub.zmax -B.sub.zmin =2.epsilon..multidot.B.sub.o
This type of magnetic structure is in particular that described in "Journal de physique lettres", and does not make it possible to separate isotopes while: EQU .DELTA.m/m&lt;.DELTA.B/B.sub.o =2.epsilon. (1)
In this relation .DELTA.m=m1-m2 represents the difference of the masses m1 and m2 of two isotopes of a mixture, m being the mean mass of two isotopes of the mixture while m=(m1+m2)/2. When the relation (1) is satisfied, the heating of the resonant isotope in the magnetic field is inadequate to expect a correct isotope separation.
The journal "Journal de physique lettres", shows that for resonant ions describing a spiral trajectory of radius R and called the "Larmor radius", account is not generally taken of the increase in the Larmor radius. For example, this is the case in the apparatus described in "Proceedings contributed papers", vol. II, International Conference on Plasma Physics, LAUSANNE, June 27-July 3 1984, p 365. In a tight cylindrical enclosure and along the axis thereof, said apparatus comprises an ion or plasma source and a regular magnetic structure having several regularly spaced magnetic rings along the axis of the enclosure.
In this apparatus, as the magnetic rings are regularly spaced along the enclosure axis, no account is taken of the increase in the Larmor radius and consequently of the initial perpendicular velocity of the ions. Thus, this apparatus, which also has no ion collection system, cannot be used for isotope separation.
The object of the invention is to obviate these disadvantages and in particular to provide an apparatus in which the magnetic assembly is constituted by several magnetic rings or by a spiral winding around the axis of the enclosure, the distribution of the magnetic rings or the spacing between the turns of the magnetic winding obeying a predetermined law taking account of the increase in the Larmor radius and the initial perpendicular velocity of the ions emitted by the source. Thus, this apparatus makes it possible to obtain a correct isotope separation.